Semiconductor light emitting diode (LED) devices, which are primarily inorganic, have been made since the early 1960's and currently are manufactured for usage in a wide range of consumer and commercial applications. The layers comprising the LEDs are based on crystalline semiconductor materials. These crystalline-based inorganic LEDs have the advantages of high brightness, long lifetimes, and good environmental stability. The crystalline semiconductor layers that provide these advantages also have a number of disadvantages. The dominant ones have high manufacturing costs; difficulty in combining multi-color output from the same chip; efficiency of light output; and the need for high-cost rigid substrates.
In the mid 1980's, organic light-emitting diodes (OLEDs) were invented (Tang et al, Applied Physics Letter 51, 913 (1987)) based on the usage of small molecular weight molecules. In the early 1990's, polymeric LEDs were invented (Burroughs et al., Nature 347, 539 (1990)). In the ensuing 15 years organic-based LED displays have been brought out into the marketplace and there has been great improvements in device lifetime, efficiency, and brightness. For example, devices containing phosphorescent emitters have external quantum efficiencies as high as 19%; whereas, device lifetimes are routinely reported at many tens of thousands of hours. However, in comparison to crystalline-based inorganic LEDs, OLEDs suffer reduced brightness, shorter lifetimes, and require expensive encapsulation for device operation.
To improve the performance of OLEDs, in the late 1990's OLED devices containing mixed emitters of organics and quantum dots were introduced (Mattoussi et al., Journal of Applied Physics 83, 7965 (1998)). Quantum dots are light-emitting nano-sized semiconductor crystals. Adding quantum dots to the emitter layers could enhance the color gamut of the device; red, green, and blue emission could be obtained by simply varying the quantum dot particle size; and the manufacturing cost could be reduced. Because of problems such as aggregation of the quantum dots in the emitter layer, the efficiency of these devices was rather low in comparison with typical OLED devices. The efficiency was even poorer when a neat film of quantum dots was used as the emitter layer (Hikmet et al., Journal of Applied Physics 93, 3509 (2003)). The poor efficiency was attributed to the insulating nature of the quantum dot layer. Later the efficiency was boosted (to ˜1.5 cd/A) upon depositing a mono-layer film of quantum dots between organic hole and electron transport layers (Coe et al., Nature 420, 800 (2002)). It was stated that luminescence from the quantum dots occurred mainly as a result of Forster energy transfer from excitons on the organic molecules (electron-hole recombination occurs on the organic molecules). Regardless of any future improvements in efficiency, these hybrid devices still suffer from all of the drawbacks associated with pure OLED devices.
Recently, a mainly all-inorganic LED was constructed (Mueller et al., Nano Letters 5, 1039 (2005)) by sandwiching a monolayer thick core/shell CdSe/ZnS quantum dot layer between vacuum deposited inorganic n- and p-GaN layers. The resulting device had a poor external quantum efficiency of 0.001 to 0.01%. Part of that problem could be associated with the organic ligands of trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP) that were reported to be present post growth. These organic ligands are insulators and would result in poor electron and hole injection onto the quantum dots. In addition, the remainder of the structure is costly to manufacture, due to the usage of electron and hole semiconducting layers grown by high-vacuum techniques, and the usage of sapphire substrates.
As described in co-pending, commonly assigned U.S. Ser. No. 11/226,622 by Kahen, which is hereby incorporated by reference in its entirety, additional conducting particles may be provided with the quantum dots in a layer to enhance the conductivity of the light-emitting layer.
Quantum dot light-emitting diode structures may be employed to form flat-panel displays. Likewise, colored-light or white-light lighting applications are of interest. Different materials may be employed to emit different colors and the materials may be patterned over a surface to form full-color pixels. In various embodiments, the quantum dot LEDs may be electronically or photonically stimulated and may be mixed or blended with a light-emitting organic host material and located between two electrodes.
Referring to FIG. 13, a prior-art structure employing electronic stimulation uses a substrate 10 on which is formed a first electrode 12, a light-emissive layer 33 of quantum dots 18 dispersed in an organic light-emitting medium 31, and a second electrode 16. Upon the application of a current from the electrodes, electrons and holes injected into the matrix create excitors that are transferred to the quantum dots for recombination, thereby stimulating the quantum dots to produce light. Such a design is described in WO 2005/055330, by Hikmet et al., published Jun. 16, 2005. P-type and/or an n-type organic transport, charge injection, and/or charge blocking layers 22 and 24 respectively may be optionally employed to improve the efficiency of the device. Typically, one electrode will be reflective (e.g. second electrode 16) while the other may be transparent (e.g. first electrode 12). No particular order is assumed for electrodes 12 and 16, although they are referenced throughout in this document as first and second, respectively.
While quantum dots may be useful and stable light emitters, in prior-art designs the emitted light may be trapped within the light-emitting structure employed to provide current or photo-stimulation to the quantum dots. Due to the high optical indices of the materials used, most of the photons generated by the recombination process are actually trapped in the devices due to total internal reflection. These trapped photons never leave the devices and make no contribution to the light output from these devices. Because light is emitted in all directions from the light-emitting layer, some of the light is emitted directly from the device, and some is emitted into the device and is either reflected back out or is absorbed, and some of the light is emitted laterally and trapped and absorbed by the various layers comprising the device. In general, up to 80% of the light may be lost in this manner.
In the prior-art example of FIG. 13, electrode 12 is transparent and may be typically formed from metal oxides such as indium tin oxide (ITO) having an optical index of 1.8-2.0. Light-emitting organic materials 31 have optical indices of approximately 1.7. P-type and/or n-type organic transport layers 22 and 24, respectively, optionally employed to improve charge injection, typically have optical indices of approximately 1.65-1.7 for organic materials; inorganic materials typically have indices greater than or equal to 2.0. Substrates on which light-emitting devices are formed typically comprise glass or plastic, having an optical index of approximately 1.5.
Light emitted in a high-index layer will be trapped due to total internal reflection when the light encounters a low-index layer. Referring to FIG. 14, a prior-art light-emitting device has a transparent substrate 10 with a relatively low optical index, a first transparent electrode 12 having a relatively higher optical index, a light-emitting layer 33 having a relatively higher optical index, and a reflective second electrode 16. Some light emitted from the light-emitting layer 33 will be emitted directly out of the device, through the substrate 10, as illustrated with light ray 1. Other light may also be emitted and internally guided in the substrate 10 and light-emitting layers 33, as illustrated with light ray 2. Alternatively, some light may be emitted and internally guided in the light-emitting layer 33 and the first transparent electrode 12, as illustrated with light ray 3. If the light-emitting layer 33 has an optical index higher than the optical index of the transparent electrode 12, light may also be trapped in the light-emitting layer 33 alone (see, e.g., light ray 4). Light rays 5 emitted toward the reflective second electrode 16 are reflected by the reflective second electrode 16 toward the substrate 10 and then follow one of several light ray paths 1, 2, 3, or 4. Similar light trapping occurs with relatively high-index optional charge-injection or charge-transport layers (not shown in FIG. 14).
There is a need therefore for an improved inorganic light-emitting diode device structure that improves the efficiency of the light-emissive display device by releasing more of the heretofore trapped light in the display device.